Symmetry and coplanarity of organic molecules affect their packing and photovoltaic properties in solution-processed solar cells

Is preservation of symmetry necessary for coarse-graining?

This work investigates if preserving the symmetry of the underlying molecular graph of a given molecule when choosing a coarse-grained (CG) mapping significantly affects the CG model accuracy.

Synthesis of an indacenodithiophene-based fully conjugated ladder polymer and its optical and electronic properties

A fully conjugated ladder polymer (PFIDT) based on the indacenodithiophene unit was synthesized via a simple strategy.

Applying Thienyl Side Chains and Different π-Bridge to Aromatic Side-Chain Substituted Indacenodithiophene-Based Small Molecule Donors for High-Performance Organic Solar Cells

A pair of linear tetrafluorinated small molecular donors, named as ThIDTTh4F and ThIDTSe4F, which are with tetrathienyl-substituted IDT as electron-rich central core, electron-deficient difluorobenzothiadiazole as acceptor units, and donor end-capping groups, but having differences in the π-bridge (thiophene and selenophene), were successfully synthesized and evaluated as donor materials in organic solar cells. Such π-bridge and core units in these small molecules play a decisive role in the formation of the nanoscale separation of the blend films, which were systematically investigated through absorption spectra, grazing incidence X-ray diffraction pattern, transmission electron microscopy images, resonant soft X-ray scattering profiles, and charge mobility measurement. The ThIDTSe4F (with selenophene π-bridge)-based device exhibited superior performance than devices based on ThIDTh4F (with thiophene π-bridge) after post annealing treatment owing to optimized film morphology and improved charge transport. Power conversion efficiency of 7.31% and fill factor of ∼0.70 were obtained by using a blend of ThIDTSe4F and PC₇₁BM with thermal annealing and solvent vapor annealing treatments, which is the highest PCE from aromatic side-chain substituted IDT-based small molecular solar cells. The scope of this study is to reveal the structure-property relationship of the aromatic side-chain substituted IDT-based donor materials as a function of π-bridge and the post annealing conditions.

Dithienobenzodithiophene-Based Small Molecule Organic Solar Cells with over 7% Efficiency via Additive- and Thermal-Annealing-Free Processing

Here we introduce a novel small molecule based on dithienobenzodithiophene and rhodanine, DTBDT-Rho, developed to study the effect of the rhodanine substitutuent on small molecule bulk heterojunction (BHJ) solar cells. DTBDT-Rho possesses distinct crystalline characteristics, sufficient solubility in chlorinated solvents, and broad absorption properties. Therefore, solution-processed BHJ photovoltaic cells made with DTBDT-Rho:PC₇₁BM blends showed an extremely high power conversion efficiency (PCE; 7.10%); notably, this PCE value was obtained without the use of additives or thermal treatments. To our knowledge, the PCE over 7% is a significantly powerful value among rhodanine-based small molecule BHJ solar cells without additives or thermal treatments.

Modulating PCBM-Acceptor Crystallinity and Organic Solar Cell Performance by Judiciously Designing Small-Molecule Mainchain End-Capping Units

In this article, we report that the bulk-size and electron-donating/electron-accepting nature of moieties, which are end-capping onto small-molecule donor mainchain, not only modulate the donor's absorption, molecular frontier orbitals, and phase ordering, but also effectively tune the PC₇₁BM-acceptor phase crystallinity. Compared to the electron-deficient trifluoromethyl (SM-CF₃) units on the diketopyrrolopyrrole (DPP) small molecule mainchain ends, the electron-rich methoxyl (SM-OCH₃) units ending on the same mainchain help improve the PC₇₁BM-acceptor phase short-range ordering. As a result, the -OCH₃ capping small-molecule displays larger short-circuit current density (J_sc) when blended with PC₇₁BM (10.72 ± 0.22 vs. 16.15 ± 0.53 mA/cm²). However, the electron-donating nature of -OCH₃ raises the donor HOMO level, which leads to a quite small open-circuit voltage (V_oc) (0.624 vs. 0.881 V). Replacement of the -OCH₃ with the large and weak electron-donating aromatic carbazolyl (SM-Cz) ones affords the small molecule of SM-Cz. The SM-Cz:PC₇₁BM system affords a high V_oc of 0.846 V and a large J_sc of 13.33 ± 0.34 mA/cm² after thermal annealing, and hence gives a larger power conversion efficiency (PCE) of 6.26 ± 0.13%, which is among the top values achieved so far from the DPP molecules. Taken together, these results demonstrate that engineering the end-capping units on small-molecule donor mainchain can effectively modulate the organic solar cell performance.

Side-chain Engineering of Benzo[1,2-b:4,5-b']dithiophene Core-structured Small Molecules for High-Performance Organic Solar Cells

Three novel small molecules have been developed by side-chain engineering on benzo[1,2-b:4,5-b’]dithiophene (BDT) core. The typical acceptor-donor-acceptor (A-D-A) structure is adopted with 4,8-functionalized BDT moieties as core, dioctylterthiophene as π bridge and 3-ethylrhodanine as electron-withdrawing end group. Side-chain engineering on BDT core exhibits small but measurable effect on the optoelectronic properties of small molecules. Theoretical simulation and X-ray diffraction study reveal the subtle tuning of interchain distance between conjugated backbones has large effect on the charge transport and thus the photovoltaic performance of these molecules. Bulk-heterojunction solar cells fabricated with a configuration of ITO/PEDOT:PSS/SM:PC₇₁BM/PFN/Al exhibit a highest power conversion efficiency (PCE) of 6.99% after solvent vapor annealing.

Theoretical investigations of the small molecular acceptor materials based on oligothiophene – naphthalene diimide in organic solar cells

The electronic transmission paths of NDI-T3DCRD with the centroid distance from core molecule to ambient molecules marked.

Effective D-A-D type chromophore of fumaronitrile-core and terminal alkylated bithiophene for solution-processed small molecule organic solar cells

A new and novel organic π-conjugated chromophore (named as RCNR) based on fumaronitrile-core acceptor and terminal alkylated bithiophene was designed, synthesized and utilized as an electron-donor material for the solution-processed fabrication of bulk-heterojunction (BHJ) small molecule organic solar cells (SMOSCs). The synthesized organic chromophore exhibited a broad absorption peak near green region and strong emission peak due to the presence of strong electron-withdrawing nature of two nitrile (–CN) groups of fumaronitrile acceptor. The highest occupied molecular orbital (HOMO) energy level of –5.82 eV and the lowest unoccupied molecular orbital (LUMO) energy level of –3.54 eV were estimated for RCNR due to the strong electron-accepting tendency of –CN groups. The fabricated SMOSC devices with RCNR:PC₆₀BM (1:3, w/w) active layer exhibited the reasonable power conversion efficiency (PCE) of ~2.69% with high short-circuit current density (J_SC) of ~9.68 mA/cm² and open circuit voltage (V_OC) of ~0.79 V.

Structure-property relationships: asymmetric alkylphenyl-substituted anthracene molecules for use in small-molecule solar cells

Two asymmetric anthracene-based organic molecules, NDHPEA and TNDHPEA, were prepared without or with a thiophene spacer between the anthracene and naphthalene units. These asymmetric oligomers displayed different degrees of coplanarity, as evidenced by differences in the dihedral angles calculated by using DFT. Differential scanning calorimetry and XRD studies were used to probe the crystallization characteristics and molecular packing structures in the active layers. The coplanarity of the molecules in the asymmetric structure significantly affected the crystallization behavior and the formation of crystalline domains in the solid state. The small-molecule crystalline properties were correlated with the device physics by determining the J-V characteristics and hole mobilities of the devices.

Side chain structure affects the molecular packing and photovoltaic performance of oligothiophene-based solution-processable small molecules

Small molecules with alkyl side chains of different lengths were prepared with 2,2′-bithiophene, terthiophene and thiobarbituric acid as the central core, spacer and end-cap. Uniform, shorter chain lengths gave stronger intermolecular interactions, favoring crystallization.

Effects of end-capped acceptors subject to subtle structural changes on solution-processable small molecules for organic solar cells

Three subtle structural changed end-capped acceptors with increased electron withdrawing ability were synthesized and introduced into a planar conjugated backbone. The relationship between molecules bearing the three end-capped acceptors and their photovoltaic properties was studied.

Small molecules based on thieno[3,4-c]pyrrole-4,6-dione for high open-circuit voltage (VOC) organic photovoltaics: effect of different positions of alkyl substitution on molecular packing and photovoltaic performance

Two different thienopyrroledione (TPD)-based small molecules (SMs) with different alkyl substitution positions were synthesized, and their photovoltaic properties are measured and compared to examine the effect of the alkyl substitution position on their optical, electrochemical, and photovoltaic properties. The use of TPD as an electron-accepting unit in conjugated SMs effectively lowers the highest occupied molecular orbital (HOMO) energy levels of the conjugated SMs and leads to high open-circuit voltage (VOC). The two SMs with n-hexyl group substituted at different positions exhibit almost identical optical and electrochemical properties in the pristine state. However, the crystallographic and morphological characteristics of the two SMs are significantly different, because they are blended with PC71BM. The SM in which n-alkyl groups are substituted at the central accepting unit exhibits a power conversion efficiency (PCE) of 6.0% with VOC=0.94 V, which is among the highest PCE values of TPD-based SM devices, whereas the SM with n-alkyl groups being substituted at the chain ends shows a moderate PCE value of 3.1%.